I like the Elvis Costello one which starts 'I knew of 2 sisters whose name it was Christmas, and one was called Dawn of course....' Cant remember the title, and I also like Fairytale of New York and Rebel Jesus and for a bit of tradition, O Come O come Emmanuel.
I only know one verse of a particularly rude song which makes me laugh 'ho, ho fuckin' ho, what a crock of shit.....' A friend sent me an animated version with a santa with a broad Australian accent and it still cracks me up! (I'll send it on if you pm me with your email address but it is a tad rude!)